The present invention relates to papermaking machines used in forming tissue grades of paper in general and to machines employing crescent formers and twin wire formers in particular.
Tissue is a lightweight grade of paper which is formed, pressed and dried on a single large dryer called a Yankee dryer. The tissue is creped from the surface of the Yankee dryer by a doctor blade creating a soft absorbent tissue. Tissue is widely used in products such as toilet paper, paper towel, napkins, and facial tissues.
A typical tissue forming machine consists of a fourdrinier former which transfers the web to a press felt which conducts the tissue web through a pressing section. Following the pressing section the web is pressed against a Yankee dryer roll to firmly adhere the web to the surface of the roll. The transfer roll may have a nip loading of approximately 200 pounds per linear inch and sometimes various sprays are used to increase adherence to the surface of the Yankee dryer roll. The tissue web is rapidly dried on the surface of the Yankee dryer which may have a diameter of over 22 feet. The dried web is scraped off the surface of the dryer by a doctor blade in a process known as creping which increases the bulk and absorbency of the tissue.
When attempting to modernize a typical older tissue making machine, the forming section, which has limited water handling capacity, which impacts formation potential, and needs substantial upkeep due to age, is advantageously replaced with a modern twin wire former or crescent former. Updating the pressing section, however, presents problems: modern fabrics are stiff and cannot be bunched up and threaded through the threading openings on older machines.
The existing pressing section typically cannot sufficiently reduce the water content of the higher speed web produced by the new crescent or twin-wire former. Because drying capability is limited by the maximum size and temperature of the Yankee dryer, if the performance of older machines is to be increased without increased drying capability, a solution which improves the capacity of the pressing section must be found. At the same time, it is necessary to eliminate the long spans where the web is supported by a felt, which have the potential for sheet drop-offs and hole formation.
What is needed is a combination of tissue forming section and press section which can be used to improve the performance of existing tissue making machines.